fix x86_64 lrintl asm, again

the underlying problem was not incorrect sign extension (fixed in the
previous commit to this file by nsz) but that code that treats "long"
as 32-bit was copied blindly from i386 to x86_64.

now lrintl is identical to llrintl on x86_64, as it should be.

math: fix lrintl.s on x86_64 (use movslq to signextend the result)

x86_64 math asm, long double functions only

this has not been tested heavily, but it's known to at least assemble
and run in basic usage cases. it's nearly identical to the
corresponding i386 code, and thus expected to be just as correct or
just as incorrect.